"The recommendation used to be that one should ride a tri/TT frame that's a size smaller than one's road frame. Is this still true?"
This principle used to be true when there were only road frames on the market. It used to be that one might get a smaller frame for TTs back when tri/TT bike geometry wasn't worked out all that well--a smaller frame would enable one to get lower in front. Today, there really isn't much comparison in terms of nominal frame sizes, as each manufacturer seems to have its own way of measuring its products. Stack (how high a bike might be) and reach (how long a bike might be) is the more common basis of measurement in the tri world, and it makes sense for TT and road bikes, too.
At any rate, there are enough design and geometry difference between frames these days that you need to be careful as to what you select . Ideally, you should get a proper fit or try out the bikes that interest you. For example, I need to ride TT frames that are long (think "stretched") and low (think short head tubes) for me to get comfortable. Because of my build, I've found that a 51cm Cevelo P3 and a 48cm Quntana Roo CD 0.1 fit me best. Note that my position is identical on both bikes, despite the three cm difference in frame sizes.
Now, bikes that are shorter and taller (think Trek, for instance) won't work for me, as I'd need to get a stem with a good amount of drop in order to get low enough; the stem also would have to be relatively long, too, for me to get enough reach. While I could get a Trek to work for me, it wouldn't be ideal.
Add in another dimension--the aerodynamic design of the frame itself. A cylinder isn't very aero; but many riders will place a tall sack of spacers on top of their headsets in order to get their bars high enough. So, all of the engineering design that went into sculpting a frame's head tube is thrown away because the rider did not match the frame's design to their bar's height requirements. Today, there's a school of thought that suggest that one should use the *largest* frame size possible, while still optimizing fit, so that one does not need to make so many aerodynamic compromises.
And factor in other variables, such as aerobar design--will the aerobars sit high over the base bars (like some versions of 3T and Profile), or are they relatively low profile (like Hed, Zipp, Vision, etc.). Your aerobars also need to be matched to your frame.
It all can be complicated; some rules of thumb might get you started, but one needs to be attentive to details in today's market.
Here's a range of some of my frames:
I go smaller in my TT frames, but only because I need the drop and reach. Your situation might be very different.
Don: Cycling Tech >